Although reduced glutathione (GSH) is known to have an inhibitory (antioxidant) effect on lipid peroxidation in microsomes from liver in vitro and is thought to play an important role in controlling lipid peroxidation in vivo through the action of glutathione peroxidase, it was observed that, in contract, GSH dramatically enhanced NADPH-dependent lipid peroxidation in rodent kidney microsomal fractions. This unexpected result was studied further with the aim of understanding the mechanism by which GSH and other thiols promoted lipid peroxidation. This process could have toxocological importance in kidney in that it may amplify NADPH-dependent lipid peroxidation.